I'm running a dnd session for 8 players (please don't tell me that is too much, I know that, but this is not the point) a few sessions in, I realize that one of the characters is an evil alignment, and is doing everything they can to cause havoc. Some examples are:

walked up to a Npc that was supposed to be a major part of the story and started slapping them. Causing me to rewrite the entire campaign

Getting the party arrested, causing me to rewrite the campaign.

Any tips? I don't want to railroad the campaign, but I can't let my hard work go to waste

It sounds like you could be writing too much?  As a DM, I never have any more of the story ready that from what I judge to be likely to happen next session.  Planning any further than that is futile if you aren’t going to railroad.  

I bring this up because you are saying “causing me to rewrite the entire campaign”… Reducing the amount you write could lead to a lot less frustration on your part, especially with a large and unpredictable 8 person group. 

And regarding the group size, through the sheer law of averages you are more likely to have a problem player, so this should have been something you were prepared for- try adding more structure and rigidity to the game.  Even during free form RP, everyone should be in an initiative-like turn order where this person speaks for a few minutes, then this person, then this person… then we move on to the dungeon or whatever it is next.  Anyone speaking or acting out of turn gets ignored and whatever it was never happened.  

It really is the only way to run a large group assuming you don’t have exceptionally well behaved players, and will at least put a spotlight on the bad behavior from the problem player.  This is where I would start. 

Just from the examples you've given, this doesn't sound like their alignment is the problem. It sounds like a player problem.  Now, they might be using 'I'm evil' to justify their actions, but it's not really about that.  They sound like they are just causing chaos for the sake of chaos. 

Players tend to be chaos gremlins for a few reasons:

- They are bored.  Especially with a large group, players with shorter attention spans for listening to their companions RP tend to cause chaos just to do something. 

- They think it's funny.  Some players see the game world as their own personal sandcastle to stomp on.  They think of it as a cartoon where the person that does the most outrageous thing gets the best stories to tell. 

- They don't understand the game.  Some new players especially don't fully understand how to RP a character.  They are presented with unlimited freedom for the first time in their lives and they test the limits like a toddler.  That's not meant to be an insult.  They are just new and trying to figure it all out.  They aren't thinking about the PCs and NPCs as living people with their own thoughts and feelings. They are thinking of them as toys to play with.  If their primary understanding of games comes from video games especially, a tabletop RPG is incredible freedom, but they aren't used to lasting consequences. In a video game, NPCs usually forget what you do to them immediately.  And you can always reset the game from your last save.

- They see the game as a form of stress relief.  Their own real lives are so restricted that being able to slap a person without any real consequences, even if it's pretend, is cathartic.

- They actually think they are playing their character well.  They make every decision based on 'what my character would do.'  But they usually aren't thinking as deeply about it as they think they are.  Because it's very rare for any realistic person to cause utter chaos. Or they haven't considered that maybe some characters don't belong in an adventuring party.  They aren't thinking about the other players, and how their actions might be causing them grief.

So first, take SireSamuel's advice. Try not to write ahead so much that you don't know what to do if a player slaps an NPC.  Give yourself a break.  Make a general outline of what is happening in the world.  And allow yourself freedom to get the players to their destinations in a number of ways. 

Second, the more your NPCs act like real people, the more the player will understand.  Real consequences for their actions will either make them tone it down or decide its best to start over.  The party won't tolerate it long if they keep getting every NPC relationship ruined and every town puts up wanted posters.  But just punishing them all the time isn't the best solution, so that's why the next part matters...

Third, figure out what is making this player act like a chaos gremlin.  If you can address the source of the problem, you can fix it better.  You can solve problems like boredom by giving them character specific things to do, somewhere safe where they won't break things.  You can give them outlets for frustration that help your campaign rather than hurt it.  You can give them despicable villains to humiliate.  You can give them stuffy nobles to slap and have the crowds cheer them for it.  You can help them if their character concept isn't meshing with the rest of the party.  You can suggest ideas for them to get what they want out of the game without ruining the fun of the other players. 

Good luck!

I agree with SireSamuel. Especially with a group of that size, you have to release the reins a bit and learn to be flexible. Consider running a West Marches campaign, rather than an elaborate story. Aside from that, this is why Session Zero is so important. It is worth establishing before the first session that the players have a reason to cooperate and work toward a common goal. However, if all else fails, you'll simply need to talk to the problem player privately. Let them know that their playstyle isn't compatible with the game you would like to run, and ask if they'd be willing to either tone it down, or switch to a new character that has a different personality.

As the DM, you fill a special role, which comes with certain privileges, however, the best DMs build a world that grows around the players, rather than building a world that the players are merely actors within. Have an outline of where you think things may go, with an understanding that the script was never meant to be followed.

I always tell my players during session zero that they are the "good guys" - they don't have to be of a good alignment, but they have to help move the adventure along by defeating or marginalizing the foes in the various encounters.

At this point, all you can do is explain that his PC's actions are disruptive to the campaign, and ask him to stop.

In future campaigns,  explain to the players at session zero, that everyone in the party is supposed to be good-aligned, and cooperate ,and work as a team.

By the way, slapping NPCs isn't really an Evil act, it is a Chaotic act.

What prevented the NPC from getting the character arrested (or otherwise removed from the vicinity) for personal assault?

Quote from dragon8824 >>

I'm running a dnd session for 8 players (please don't tell me that is too much, I know that, but this is not the point) a few sessions in, I realize that one of the characters is an evil alignment, and is doing everything they can to cause havoc. Some examples are:

walked up to a Npc that was supposed to be a major part of the story and started slapping them. Causing me to rewrite the entire campaign

Getting the party arrested, causing me to rewrite the campaign.

Any tips? I don't want to railroad the campaign, but I can't let my hard work go to waste

 

 Echoing some of what others have said, you do have a player problem. Players don't have to create good aligned characters. They have to create characters that will cooperate with the group, work as a member of a team, and integrate into the world and the plot. Good has nothing to do with any of this. If your player gives you the 'wangrod defense', it might be past time to have a face-to-face chat about socially acceptable behavior and cooperative game play.

Second the idea that you only build two horizons out, and that the players need to declare what directions and decisions they want to engage in at the end of current session, so that you can prepare for the next session. If they, as a group or one in particular, decides that they want to change their mind last minute with no notice, that's fine. Call the game to a close, as you don't have that new chain prepared for them. They make a social contract to go a certain direction, and take on a certain task, which you prep. Should that change, nothing else will be prepared for them to explore. SlyFlourish - Thinking Two Horizons Out.

Lastly, the plot, or main driving force in your game won't necessarily be put on pause because the party is being jailed somewhere for a month. The march of the BBE continues inexoribly toward it's conclusion. The party is 'supposed' to be there to stop it, or slow it down long enough to get something/someone that IS strong enough to stop it. They cannot perform their tasks, if they are jailed. Their effectivess is diminished if the local authorities are trying to aprehend them, and will not work with them diplomatically. To combat this, arrest the PC in question, and fast-forward the timeline of the game by a week, month, whatever seems appropriate. Just enough time needs to pass that shows a change in the game world, and now the party is no longer enjoying the luxury of time. They are behind the power-curve of the plot, and need to work (and here's the important bit) TOGETHER to achieve their goals. Don't re-write anything. They want to be lounging in the dungeons as a prisoner? The BBE wants them there too! Let them fail, maybe fail forward, but let them fail. Your player is treating your game like a video game, testing the limits of the game engine and the game world. Show them those limits. Let them see the Game Over You Lose screen before the credits start to roll. Asking the ever-famous: "Are you sure you want to do that?" tends to make players re-think courses of action. The signs that their failure means something can reinforce the consequence that their decisions matter to the outcome of the game, both Good and Bad. Show, don't tell, them the results of their actions and choices.

I’ve always wanted to play an evil character but I doubt I could pull it off without being good. In my group we have a few players who have also been DMs. We have an understanding that if the party is together they have a reason to be there, otherwise make a new character. Definitely sounds more like a player problem or someone who think chaotic =evil. I saw a good video where the evil player did have their own goals but to achieve them they needed the party, so the player saw the party as pions then companion and finally friends. They did evil stuff similar to how a rogue might steal or a bard might be horny, but if someone messed with the party then the evil player would intervene because it ultimately messed with their longtime plans. 

Also sometimes the players need to understand consequences, so if a player slaps an important NPC maybe it starts a fight, maybe they end up in prison and end up paying a fine. It’s all role playing, in the Waterdeep campaign it’s advised to give the party the “code legal” where killing results in execution etc. so basically as DM you’re saying that uf the players messes up there are consequences and they may be killed. Alternatively if the player/character is liked then party may reign them in for messing up plans and getting NPCs killed. Or they may plan a prison break, to save the player.

this is all in game stuff, assuming everyone is having fun, as everyone has probably heard a 100 times if something is adversely affecting the game and EVERYONE having fun, then have a chat out of game 

There are two steps here:

1) Tell the player that you are frustrated with his character's antics, and that it is ruining the fun for you as a DM. Ask him why he is doing this and what you can do to make it more fun for him, so he does not have to create fun by creating problems for the rest of the party. Take his feedback into account and don't get defensive about it, but also make it your most important note that you just can't deal with his character derailing his campaign any more, and that he has to stop it.

2) If your player continues his behavior despite numerous warnings, it's okay to kick him out of the campaign. You as a DM deserve to have some fun too, and your players shouldn't be able to ruin that for you. If you want to remain friends with your problem player, or at least maintain an amicable relationship with them, then you can easily do that while gently removing them from your campaign as long as you are polite and kind about it.

And as a sidenote, I know a lot of people on the forums like blaming the alignment system for things like this. But alignment isn't at fault here. You can still play a crazy evil player and derail other peoples campaigns without bothering to even mark down that your character is evil.

When I had a player who was playing his PC that way I told him privately not to and I added in game consequences. His character did something rude to a village elder and suddenly he had 20+ crossbows pointed at him and he was kicked out of the village.